Maand: augustus 2016

There is an epidemic of mosquito and tick borne diseases spreading among humans. Yellow fever is returning, west nile fever has increased, dengue fever has increased 30-fold between 1960 and 2010 and Zika turned into a brain-damaging disease seemingly out of nowhere. There’s also malaria, which most people are well familiar with.

What causes this problem? There are a few important factors. There is more international travel, at faster paces, than ever before in human history. The natural barriers between human communities that would normally prohibit the spread of disease no longer exist. There are also more humans out there. Diseases that would normally drop below the threshold needed for them to survive due to low population density now manage to find sufficient human hosts to proliferate. One study found a minimum population density of roughly five people per square kilometer for malaria to gain a foothold in a particular part of the Amazon.

Most importantly however, for mosquitoes and ticks to be good transmitters of viruses and other pathogens, they have to be exposed to a limited number of species for them to prey on. Low biodiversity means that it becomes easy for a virus or bacteria to jump from one tick or mosquito to his victim, then from his victim to new ticks or mosquitoes. If the animal that’s bitten is too different from the animals that the pathogen normally replicates in, the tick can continue his life cycle by sucking up blood, but the disease can’t proliferate in the victim. As a result, mosquitoes and ticks are born that have been cleansed from the disease. These animals, that put a stop to the life cycle of the pathogen, are known as “dead end hosts”.

It’s important to have a lot of different types of dead end hosts, if we wish to reign in the spread of vector-borne diseases. Large animals, like deer and carnivores, tend to be relatively poor hosts for these diseases. For Lyme disease, the best host is the white-footed mouse. It’s generally found that places with a lot of different types of rodents have far fewer cases of Lyme disease in humans, than places where species diversity is low.

The king of all dead end hosts however, appears to be the humble lizard. One reason for this is thought to be that many lizards are capable of surviving winter with a below-zero body temperature. During this period, the body of the lizard is cleansed of pathogens like Lyme disease. As a result, studies tend to find that ticks found on lizards are much less likely to carry Lyme disease than ticks found elsewhere. Just 1.4% of ticks on lizards in the Netherlands carry Lyme disease, compared to 24% of ticks found in heath and 19% of ticks found in forests. In the United States, a similar effect is found.

There are different perspectives on biodiversity. Deep ecologists would say that non-human life has a right to survive undisturbed in spite of whatever benefits these organisms may have for humans. Unfortunately, such arguments tend not to be particularly persuasive for politicians like Paul LePage, who argues that “Job creation and investment opportunities are being lost because we do not have a fair balance between our economic interests and the need to protect the environment”.

Because the survival of non-human species needs to deliver economic benefits for most policymakers to show interest, more ecologists these days study the impact of biodiversity on disease burdens in humans. Perhaps the realization that a world completely dominated by humans and a few species that happen to escape our destructive impact is also a world where humans are plagued by a growing number of vector-borne diseases could help a few species escape the threat of extinction that looms over them.

It’s quite well known that the growth in human height in most developed countries has stagnated. This is easy to explain and isn’t particularly worrisome. Human height is a product of underlying genetic potential, as influenced by the environment. As the diet of a nation improves, its people’s adult height should increase, up to a limit, as when all nutrient shortages are addressed, little more gains in height are to be expected.

On the other hand, in recent years it has been observed that despite simultaneous ongoing economic development, in many countries human height is declining again. Most worrisome perhaps is the rapid pace at which human height is declining in these nations. In some countries, decades of progress has been eradicated in a few years.

Consider the case of Nigeria below:

As can be seen here, by 1996, average male height is back to a level last seen in 1941.

Ethiopia has similarly been affected by a downturn much steeper than the initial rise in height:

Egypt has similarly seen an enormous decline in height, a typical case of Ugo Bardi’s Seneca cliff:

Not all poor nations are affected, China, Brazil and many others are still booking gains in height. It is however, a problem that affects nations collectively home to hundreds of millions of people.

What could explain this mysterious problem? Economists who look at Nigeria or many of the other African nations affected by this problem would see countries that were plagued by economic stagnation for decades, but now seem to be rapidly expanding their economies.

The authors of the study suggested that changing dietary habits may play a role, as well as collapsing health care system. This seems the most likely possibility to me. In particular, it’s worth noting that people in much of Africa depend upon bushmeat as their main source of protein.

As bushmeat is depleted, people are forced fill their diet with other foods. One source of nutrition that’s very popular in Africa is cassava, which is filled with cyanide. Chronic cyanide exposure however is known to cause a decline in growth, as well as cognitive problems. None of these explanations however could possibly explain what’s happening in Egypt. Egypt has no bushmeat, nor do Egyptians eat cassava.

It’s somewhat hard to draw any strong conclusions on what could have led to this sudden and rapid reversal in human height. One factor that may be relevant is the spread of smoking in much of the third world, as it’s known that second hand smoke exposure reduces the growth rate of young children. It’s also possible that a decline in breastfeeding rates played a role.

Another factor that may be important is a rise in vitamin D deficiency. If rural women and children were more active outside, increasing urbanization may have led to an increase in vitamin D deficiency that stunts growth. It’s known that vitamin D deficiency is epidemic throughout the third world.

Finally, the main and most important factor to be considered would have to be be a drastic reduction in the quality of people’s diet. If people’s diet has become less diverse, they would be expected to suffer a variety of nutrient deficiencies that impact their growth. Fresh fruit and vegetables are difficult to store and thus rather expensive. Urbanization may lead people to grow dependent on a much worse diet.